drm/i915/wm: move ILK watermark sanitization to i9xx_wm.[ch]

Move sanitize_watermarks() to i9xx_wm.[ch] and rename as
ilk_wm_sanitize(). The slightly unfortunate downside is having to expose
intel_atomic_check() from intel_display.c, but this declutters
intel_display.c nicely.

v2:
- Move to i9xx_wm.[ch] instead of intel_wm.[ch] (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215141910.433043-1-jani.nikula@intel.com